Aux 1 Unit 11 Q6

Advantages:

1. Excellent Embedability and Conformability:

• Accommodates Misalignment: White metal is relatively soft and can conform to the shaft’s surface, allowing for slight misalignments or irregularities without causing excessive wear or damage.
• Foreign Particle Embedment: Small particles or debris that enter the bearing can become embedded in the soft white metal, preventing them from scratching or scoring the shaft.

2. Good Compatibility with Lubricants:

• Oil Retention: White metal has good oil retention properties, helping to maintain a hydrodynamic oil film between the shaft and bearing, reducing friction and wear.
• Emergency Running: In case of temporary loss of lubrication, the embedded oil in the white metal can provide some level of protection and prevent immediate seizure.

3. Relatively Easy to Repair:

• Re-metalling: Worn or damaged white metal linings can be repaired through a process called re-metalling, where the old lining is removed and a new layer of white metal is cast in place. This can extend the bearing’s life and be more cost-effective than replacing the entire bearing.

4. Good Thermal Conductivity:

• Heat Dissipation: White metal has good thermal conductivity, allowing it to dissipate heat generated by friction more effectively, reducing the risk of overheating.

Disadvantages:

1. Susceptible to Corrosion and Fatigue:

• Seawater Corrosion: White metal can be prone to corrosion, especially in seawater-lubricated stern tube bearings, leading to pitting and degradation of the bearing surface.
• Fatigue Cracking: Under cyclic loading and vibration, white metal can develop fatigue cracks, potentially leading to bearing failure.

2. Lower Load Capacity:

• Limited to Moderate Loads: Compared to some harder bearing materials, white metal has a lower load-carrying capacity. It’s generally suitable for moderate loads and might not be the best choice for very large vessels or those operating in extremely harsh conditions.

3. Sensitivity to Lubrication:

• Requires Adequate Lubrication: White metal relies heavily on a continuous and adequate supply of lubricant to maintain the hydrodynamic film and prevent excessive wear. Any disruptions in lubrication can lead to rapid bearing failure.

4. Environmental Concerns (Lead-Based White Metal):

• Toxicity: Traditional lead-based white metal contains lead, which is a toxic heavy metal. While modern regulations and practices have reduced its use, environmental concerns remain, especially in the event of bearing wear or leakage.

Conclusion:

White metal offers several advantages as a stern tube bearing material, including good embedability, compatibility with lubricants, ease of repair, and thermal conductivity. However, its susceptibility to corrosion, lower load capacity, and sensitivity to lubrication are important factors to consider when selecting bearing materials.

Modern advancements in bearing materials and designs, including the development of tin-based white metals and new synthetic materials, are continually improving the performance and longevity of stern tube bearings, making them a viable option for various marine applications.

Advantages of Plastic Stern Tube Bearings

1. Corrosion Resistance:

• Immune to Seawater Corrosion: Plastics, especially those specifically formulated for marine applications, are inherently resistant to corrosion from seawater, eliminating concerns about rust and pitting that plague metallic bearings.
• Reduced Maintenance: This corrosion resistance translates to less frequent maintenance and a longer service life compared to traditional white metal bearings that require re-metalling or replacement due to corrosion.

2. Self-Lubricating Properties:

• Reduced Friction and Wear: Many plastic bearing materials have inherent self-lubricating properties or can be impregnated with lubricants, reducing friction and wear even with water lubrication. This contributes to increased efficiency and longer bearing life.

3. Environmental Friendliness:

• No Oil Pollution: Plastic bearings, when used with water lubrication, eliminate the risk of oil pollution associated with traditional oil-lubricated bearings. This is increasingly important due to stricter environmental regulations and the need to protect marine ecosystems.

4. Lightweight:

• Weight Reduction: Plastic bearings are significantly lighter than metallic bearings, contributing to weight reduction on the vessel, which can improve fuel efficiency and stability.

5. Cost-Effectiveness:

• Lower Initial Cost: In some cases, plastic bearings can be more cost-effective than traditional metallic bearings, especially when considering the long-term savings in maintenance and oil-related expenses.

Disadvantages of Plastic Stern Tube Bearings:

1. Load Capacity:

• Limited Load-Bearing Capacity: Compared to some metallic bearings, plastic bearings may have a lower load-carrying capacity. Careful design and selection are crucial to ensure they can handle the specific loads and operating conditions of the vessel.

2. Temperature Limitations:

• Sensitive to High Temperatures: Some plastic materials can be sensitive to high temperatures, limiting their use in applications with elevated operating temperatures. Proper cooling and monitoring are essential.

3. Abrasion Resistance:

• Susceptible to Abrasive Wear: While some plastics offer good abrasion resistance, they may be more prone to wear than harder metallic bearings if exposed to abrasive particles in the water.

4. Installation and Repair:

• Specialized Techniques: Installation and repair of plastic bearings might require specialized techniques and expertise compared to traditional metal bearings.

Conclusion:

Plastic stern tube bearings offer significant advantages in terms of corrosion resistance, self-lubrication, environmental friendliness, weight reduction, and potential cost savings. However, their load capacity, temperature limitations, and abrasion resistance need to be carefully considered when selecting them for a specific application. With advancements in material technology and design, plastic bearings are becoming increasingly viable for various marine vessels, contributing to a more sustainable and efficient maritime industry.